Why this winter's snow forecasts keep flummoxing meteorologists
After a brief warmup this weekend, many hardened Chicagoans were already bracing themselves for the winter weather to return in force.
Some forecasts over the weekend reported the suburbs could get as much as 13 to 14 inches of snow by Wednesday, based on “weather models,” computer simulations based on millions of data points.
And then the patterns shifted. By Monday, the National Weather Service in Chicago was calling for 6 inches or more of snow in parts of the suburbs, and most of that was expected to be south of Interstate 80.
“It's probably one of the biggest shifts in a storm track I've ever seen in my career,” said Victor Gensini, a professor of meteorology at Northern Illinois University. “This was a 100-plus mile shift.”
By Monday evening, the weather models showed most of Central Illinois and Chicago's South suburbs targeted for the most significant snowfall. Lake and McHenry counties were expected to get an inch or so, and most of the other suburbs 2 to 5 inches, said Brett Borchardt, a National Weather Service meteorologist based in the Romeoville bureau.
“Southern Chicago and especially south of Interstate 80 are really going to feel this now. But this is a pretty interesting winter storm,” he said.
The reason this storm is so hard to predict is because it is forming over us rather than coming to the region, Borchardt said. Two weaker low-pressure systems — one from the west that passed over the Rocky Mountains and one from the north coming down from Canada — are going to meet up and cause this winter storm, meteorologists explained. Plotting and forecasting this type of weather event is tougher because of that.
This hasn't been a great winter for meteorologists. Most winter storms this year have underperformed what the professional forecasters predicted.
“We've had a comedy, for lack of a better term, of snow systems that have been very underwhelming,” Gensini said. “Look, this is no different than forecasting the stock market or elections. You have an idea of what could happen and you update those forecasts when you get more information.”
Though meteorologists constantly were adjusting their forecasts, the social media chatter about a foot or more of snow didn't quite keep up.
“The brand new run of the NAM computer model (just in) shows a MONSTER snowstorm here,” FOX 32 meteorologist Mike Caplan reported on Twitter at 8:45 p.m. Saturday. “This is still subject to change and perhaps significantly so.”
An hour and a half later, the latest model showed the storm tracking north and missing much of the Chicago area, he said on Twitter. By 1 p.m. Sunday, the “new European model” had the Chicago area back in the snow zone before an updated European model at midday Monday showed the storm's southward trend, Caplan reported.
Some Chicago meteorologists poked fun at themselves Monday over forecasts that have not lived up to the hype this winter.
“1-10 inches. Book it,” WGN meteorologist Demetrius Ivory said on Twitter with an updated weather model showing the storm moved to the south and most of the Chicago area being spared significant snow accumulation.
“LOL! This winter has been a nightmare with these storms,” FOX 32 chief meteorologist Bill Bellis replied in a tweet to Ivory.
Most forecasts are “middle-of-the-road” predictions based on different factors, Gensini said. It's merely coincidence that all of this winter's forecasts have fallen short of reality, instead of the other way around. Snowfall for the season is 2.9 inches below average at O'Hare, the National Weather Service's Chicago office reported.
“In reality, the first thing we think about is the impact of the storm on humans,” said Dave Changnon, NIU's meteorology adviser. “I'd rather go on the side of putting no one at risk rather than put anyone at risk.”